<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8"/>
  <meta name="viewport" content="width=device-width, initial-scale=1.0"/>
  <title>Proxy</title>
</head>
<body>
  <div id="app"></div>
  <button id="btn">b++</button>
  <button id="btn2">a.push(b)</button>
  <button id="btn3">a[a.length-1]=666</button>
  <script src="Proxy.js"></script>
</body>
</html>